1. Field of the Invention
The present disclosure is in the field of ripple-current reduction techniques and, more particularly, relates to the application of such techniques to power electronic circuits, particularly those of transformers.
2. Description of the Related Art
Inductors are used in many ways in power electronic converters including operation as filters, energy storage and high frequency decoupling. In most cases a desirable low frequency current and an undesirable high frequency ripple current will flow in the main inductor. This current is present due to the switching involved in the operation of power electronic circuits. An inductor may also be connected to a capacitor to create a low-pass filter to allow the flow of low frequency current and to reduce the high frequency ripple.
Transformers are used in power electronic circuits primarily for electrical isolation and/or voltage scaling. Transformers can be sufficiently modeled into a simple lumped component circuit. A power electronic circuit can be used to generate a high frequency pulse width modulation of a low frequency voltage that is fed to a transformer primary winding. The secondary winding of the transformer can then be connected to a filter comprising an inductor and a capacitor.
To simplify the circuit, the leakage inductance of the transformer can be utilized to replace the external inductor of the filter, which reduces the physical number of components in the circuit. The value of the leakage inductor can be dimensioned to that of the required external inductor so that the circuit only requires the external capacitor to complete the filter.
However, such circuitry can result in the external capacitor being large and expensive, as well as the creation of a large, undesirable phase shift between the voltage and current in the output of the circuit. An integrated transformer would have a low frequency current and a high frequency ripple current flowing in the leakage inductor. The current is present due to the high frequency pulse width modulated rectangular-wave voltage applied to the primary winding. The leakage inductor can be operably connected to the external capacitor to create a filter that allows the flow of low frequency current and to reduce AC ripple on the desired output voltage.
A critical problem that arises in such circuitry is that ripple currents in a capacitor induce heating by reason of conductor losses and dielectric losses. The heating of the capacitor in turn reduces its life expectancy. Accordingly, any means that will reduce the ripple current into the capacitor has the potential to increase the life expectancy of a system that uses the capacitor. In addition, the reduction in the ripple current can reduce the required total capacitance, which in turn can lead to a reduction in the size of the capacitor and, hence, of the system. This is conventionally achieved by the mechanism of defining a fixed allowable ripple voltage across the terminals of the main capacitor before and after the ripple current reduction. An alternative can be achieved by reducing the inductance value of the inductor and maintaining the capacitance as per the original design.
There exists techniques that can reduce the ripple voltage on a capacitor and may include an increase in the frequency of the ripple current. Unfortunately, this can also increase the stress on the capacitor more than the benefits provided by a reduction in the ripple current amplitude. This consequence follows because the losses in the capacitor are frequency dependent. In addition, an increase in the frequency applied to the transformer can significantly increase the losses in the transformer, which further results in an increase in temperature. Also, the problem is exacerbated when the power level of the converters is high.
Another method used in an attempt to reduce the ripple voltage across the capacitor terminals is by the addition of more filter components. However, since classic filter design requires that these filters carry the full power of the converter system, the cost of such additional filters outweighs the benefits. There is also difficulty in damping these complex filter arrangements. In addition, the total ripple can only be spread out between all the components. Accordingly, there is a need for a technique, which reduces or eliminates ripple current into a filter capacitor of a transformer, while also reducing or eliminating the above-noted problems.